But while the pundit class may have been watching to see who won, it’s easy to forget that many voters, consciously or not, were watching for something else.

If the two presidential candidates have to pass the commander-in-chief test, looking confident, assured and ready to serve as the leader of the free world, Biden and Ryan have to pass the heartbeat-away-from-the-presidency test. If something were to happen to the president, which man seemed like he could more immediately step into the role? That, after all, is the traditional job—and the only constitutional role, besides being president of the Senate—of the vice president: to be a credible stand-in for the chief.

For Biden, the test wasn’t as critical. He’s already been vice president for nearly four years, and at 69 years old, had age working in his favor. That said, Biden both helped and hurt himself last night. He was strong, confident, well-versed in the issues—particularly foreign policy—and did not back down at all from a fight. His best moments were toward the end of the debate, when there was far more measure to his expressions, a quiet, determined resolve to his speech, and a sense of gravitas to his words. The smirks and gestures and much-Tweeted-about laughs were not what most people would want of a vice president in a critical diplomatic setting. Still, anyone watching Thursday night also knew it was a debate, and that Biden’s job was to go on the offensive.

For Paul Ryan, meanwhile, the test really mattered. It’s easy to forget that just two months ago, when Mitt Romney named him as his running mate, 38 percent of Americans didn’t even know who Paul Ryan was. He may be the intellectual leader of his own party, but to the average American who does know him, he is also a congressman far more associated with Medicare and budget charts than with wars, foreign policy and strategy in the Middle East.

This is one reason, I’d guess, that Ryan didn’t go on the attack as much as Biden, and remained (mostly) more measured in his remarks. It seems his goal was to turn in a solid, if not stand-out, performance in which he kept his cool. Especially for someone who looks even younger than his 42 years, and who is more known for wonkish number-crunching than commanding gravitas, Ryan had to put on his most diplomatic and presidential face to pass the man-in-the-wings test. The smirks and at times defensive answers did not help him, especially sitting next to Biden’s dominating presence. But he likely passed the test, though only time (and polls) will tell how well.

As most of the post-debate chatter will note, it’s unclear whether who won or lost the vice-presidential debate will really affect the results come Election Day. It’s also hard to declare a winner when the two men had different goalposts: Biden’s job was to re-energize Democrats and let them know the campaign still has plenty of fight, while Ryan’s was to maintain the GOP’s momentum while giving those who don’t know him some confidence in his abilities should the worst happen.

In three weeks, those undecided or persuadable voters won’t remember who won and who lost the debate. But somewhere in the back of their minds, when they check one box or the other on the ballot, they may remember which man looked like he could fill in for commander-in-chief.

President Franklin D. Roosevelt delivers his first radio "Fireside Chat" in Washington in March 1933. Eight years later, Roosevelt would deliver his most historic speech to Congress a day after the Pearl Harbor attack, which he then described as "a date which will live in infamy."AP